I for one am glad In Flames is coming back(since they missed a good portion of 2011), but now let's look at bands that could do Mayhem that haven't(or atleast haven't done the whole tour):
Testament, Prong, Exodus, Arch Enemy, Soilwork, Children of Bodom, Devildriver, Municipal Waste, Periphery, The Faceless, Bury Your Dead, Dillinger Escape Plan, Black Tusk, Thy Will Be Done, The Acacia Strain, Battlecross, Converge, Skeletonwitch, Revocation, A Life Once Lost, Dirge Within, Crowbar, Darkest Hour

I mean there's just a handful, and notice none could headline a shed festival